Title

Author

Date of Award

4-2018

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Dr. Sherine Obare

Second Advisor

Dr. Ekkehard Sinn

Third Advisor

Dr. Michael Barcelona

Fourth Advisor

Dr. Charles F. Ide

Abstract

The design, synthesis, and applications of anthropogenic nanoparticles continue to rise as technologies utilizing the unique chemical and physical properties of materials with nanoscale dimensions emerge. Engineered silver nanoparticles (Ag NPs) possess antimicrobial properties toward several strains of antibiotic-resistant bacteria. The manufacturing and use of Ag NPs have led to their increased presence in the environment as pollutants. The interaction of Ag NPs with other pollutants influence their surface properties, aggregation state, stability and consequently, their toxic response toward microbial pathogens. In the present study, we examined the interaction of heavy metal ions (Cd2+, Hg2+, and Pb2+) on Ag NPs stability and antimicrobial response. Nanoparticles of Ag with a diameter of 4 nm were synthesized using a wet-chemical procedure. The Ag NPs were characterized using electron microscopy and spectroscopic techniques. The stability of the Ag NPs when brought in contact with the metal ions was examined by measuring changes in the surface plasmon resonance (SPR) as microscopic imaging. We further examined the effect of the metal ions on the toxicity of Ag NPs using Escherichia coli (E.coli) as a model for gram-negative bacteria and Staphylococcus aureus (S. aureus) as a model for gram-positive bacteria.

In this dissertation, we hypothesize that ‘the toxicity of nanoscale materials, particularly Ag NPs, is impacted by the environment in which they are in and contaminants that they interact with. The following research objectives were used to test this hypothesis:

1. Synthesis of well-defined metal nanoparticles that are prevalent in commercial products. Procedures to prepare nanoparticles with various size, morphology, and high uniformity were established.

2. Determine the stability of Ag NPs under various environmental conditions, including changes in concentration, temperature, and exposure to heavy metal ions. The stability of the particles was determined by monitoring changes in the size and shape of the nanoparticles using transmission electron microscopy (TEM) and the surface plasmon resonance using UV-visible absorbance spectroscopy.

3. Investigate the stability of Ag NPs upon exposure to Cd2+, Hg2+, and Pb2+ and natural organic matter (NOM) as well as their influence on Ag NPs antimicrobial properties.

The results showed that heavy metals influenced the adsorption, aggregation, and dissolution of Ag NPs dependent on the metal ion, exposure time and concentration. When co-exposed to bacterial strains, the presence of Ag NPs and heavy metals followed trends of Ag NPs alone, indicating that the metal ions interaction to the bacteria surface was limited. Ag NPs changed the availability of heavy metals to cells and altered individual antimicrobial toxicity. The toxicity of Hg2+ was limited in the presence of Ag NPs, while Pb2+ exposure with Ag NPs displayed higher toxicity toward bacterial strain. In the presence of NOM, the heavy metals reduced the adsorption of Ag NPs, thus making them more available to multiple bacterial cells. The results heighten the need to study the interaction of nanoparticles in combination with other contaminants and their biological response as they seldom are in the environment independently.